Heat exchanger

ABSTRACT

A heat exchanger to be used as a gas cooler has a refrigerant inlet and a refrigerant outlet provided at upper and lower header sections, respectively, of a first header tank. A value represented by a formula {(L 1 +L 2 )/2}+(T×2N) is defined as an average flow path length L 0 , where L 1  represents the total interior length of both the header sections of the first header tank, L 2  represents the interior length of the header section(s) of the second header tank, T represents the length of the flat tubes, and N represents the number of the header section(s) of the second header tank. The positions of the refrigerant inlet and outlet are determined to satisfy the relation 0.8≦LX/L 0 ≦1.2, where Lx represents a flow path length for refrigerant which flows into the upper header section from the refrigerant inlet and flows out of the refrigerant outlet.

BACKGROUND OF THE INVENTION

The present invention relates to a heat exchanger, and more particularlyto a heat exchanger that can be favorably used as a gas cooler of asupercritical refrigeration cycle in which a CO₂ (carbon dioxide)refrigerant or a like supercritical refrigerant is used.

Herein and in the appended claims, the term “supercritical refrigerationcycle” means a refrigeration cycle in which refrigerant on thehigh-pressure side is in a supercritical state; i.e., assumes a pressurein excess of a critical pressure. The term “supercritical refrigerant”means a refrigerant used in a supercritical refrigeration cycle.

A supercritical refrigeration cycle includes a compressor, a gas cooler,an evaporator, a pressure reducing device, and an intermediate heatexchanger for exchanging heat between refrigerant flowing out of the gascooler and that flowing out of the evaporator. Japanese PatentApplication Laid-Open (kokai) No. 2003-279194 discloses a heat exchangerused as a gas cooler of such a supercritical refrigeration cycle. Thedisclosed heat exchanger includes first and second header tanks disposedapart from each other and extending vertically; and a plurality of flattubes disposed at vertical intervals between the two header tanks andhaving opposite end portions connected to the respective header tanks.The first header tank includes upper and lower header sections, and thesecond header tank includes a single header section which faces thefirst and second header sections of the first header tank. A refrigerantinlet is provided at the upper header section of the first header tank,and a refrigerant outlet is provided at the lower header section of thefirst header tank. The flat tubes form upper and lower paths eachcomposed of a plurality of flat tubes arranged vertically.

Japanese Patent Application Laid-Open (kokai) No. 2004-138306 alsodiscloses a heat exchanger used as a gas cooler of such a supercriticalrefrigeration cycle. The disclosed heat exchanger includes first andsecond header tanks disposed apart from each other and extendingvertically; and a plurality of flat tubes disposed at vertical intervalsbetween the two header tanks and having opposite end portions connectedto the respective header tanks. Each of the first and second headertanks includes upper and lower header sections. A refrigerant inlet isprovided at the lower header section of the first header tank, and arefrigerant outlet is provided at the upper header section of the secondheader tank. The flat tubes form three paths arranged vertically, eachcomposed of a plurality of flat tubes arranged vertically.

However, as a result of various studies, the present invention foundthat the heat exchanger disclosed in Japanese Patent ApplicationLaid-Open No. 2003-279194 raises the following problem because therefrigerant inlet and the refrigerant outlet are provided at the centerportions of the corresponding header sections with respect to thelongitudinal direction. That is, refrigerant having flowed into theinterior of the upper header section of the first header tank via therefrigerant inlet flows into a plurality of flat tubes communicatingwith the upper header section; i.e., the flat tubes of the upper path,and flows into the interior of the header section of the second headertank via the flat tubes. The refrigerant then flows into a plurality offlat tubes communicating with the lower header section of the firstheader tank; i.e., the flat tubes of the lower path, and flows into theinterior of the lower header section of the first header tank via theflat tubes. The refrigerant then flows out of the refrigerant outlet. Aflow path length of refrigerant which passes through a flat tube of theupper path, which tube is located at a vertical position in the vicinityof the refrigerant inlet, and then passes through a flat tube of thelower path, which tube is located at a vertical position in the vicinityof the refrigerant outlet, differs from that of refrigerant which passesthrough the uppermost flat tube of the upper path and passes through thelowermost flat tube of the lower path. As a result, the flow rate andflow velocity of the refrigerant become imbalanced or non-uniform amongthe flat tubes, and the heat radiation is insufficient for use as a gascooler of a supercritical refrigeration cycle.

Like the heat exchanger disclosed in Japanese Patent ApplicationLaid-Open No. 2003-279194, the heat exchanger disclosed in JapanesePatent Application Laid-Open No. 2004-138306 also has insufficient heatradiation for use as a gas cooler of a supercritical refrigerationcycle, because the refrigerant inlet and the refrigerant outlet areprovided at the center portions of the corresponding header sectionswith respect to the longitudinal direction.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the above problem and toprovide a heat exchanger which can radiate heat in a greater quantitywhen it is used as a gas cooler of a supercritical refrigeration cycle.

To fulfill the above object, the present invention comprises thefollowing modes.

1) A heat exchanger comprising first and second header tanks disposedapart from each other; and a plurality of flat tubes disposed betweenthe first and second header tanks such that the flat tubes are spacedapart from one another in the longitudinal direction of the headertanks, opposite end portions of the flat tubes being connected to therespective header tanks, the first header tank including a plurality ofheader sections arranged in the longitudinal direction of the firstheader tank, the second header tank including a single header section ora plurality of header sections which are one fewer in number than theheader sections of the first header tank, the single header section oreach of the header sections facing two adjacent header sections of thefirst header tank, a refrigerant inlet being provided at a headersection of the first header tank located at one end with respect to thelongitudinal direction, a refrigerant outlet being provided at anotherheader section of the first header tank located at the other end withrespect to the longitudinal direction, the flat tubes being divided intogroups each of which includes a plurality of flat tubes arranged in thelongitudinal direction of the header tanks and which form paths whichare equal in number to the header sections of the first header tank,

wherein when a value represented by a formula {(L1+L2)/2}+(T×2N) isdefined as an average flow path length L0, where L1 represents the totalinterior length of all the header sections of the first header tank, L2represents the total interior length of all the header section(s) of thesecond header tank, T represents the length of the flat tubes, and Nrepresents the number of the header section(s) of the second headertank, the position of the refrigerant inlet and the position of therefrigerant outlet are determined to satisfy the relation 0.8≦LX/L0≦1.2,where Lx represents a flow path length for refrigerant which flows intothe first header tank from the refrigerant inlet, passes through all theheader sections and the flat tubes of all the paths, and flows out ofthe refrigerant outlet.

2) A heat exchanger according to par. 1), wherein the first header tankincludes two header sections, the second header tank includes a singleheader section, and the number of paths is two.

3) A heat exchanger according to par. 1), wherein the amount ofcompressor lubrication oil mixed in a refrigerant to be used is 1 wt. %or less.

4) A heat exchanger comprising first and second header tanks disposedapart from each other; and a plurality of flat tubes disposed betweenthe first and second header tanks such that the flat tubes are spacedapart from one another in the longitudinal direction of the headertanks, opposite end portions of the flat tubes being connected to therespective header tanks, the first header tank including a plurality ofheader sections arranged in the longitudinal direction of the firstheader tank, the second header tank including header sections which areequal in number to the header sections of the first header tank andwhich are arranged in the longitudinal direction of the second headertank, a refrigerant inlet being provided at a header section of thefirst header tank located at one end with respect to the longitudinaldirection, a refrigerant outlet being provided at a header section ofthe second header tank located at the opposite end with respect to thelongitudinal direction, the flat tubes being divided into groups each ofwhich includes a plurality of flat tubes arranged in the longitudinaldirection of the header tanks and which form paths which are one greaterin number than the header sections of each header tank,

wherein when a value represented by a formula {(L1+L2)/2}+{(T×(N+1)} isdefined as an average flow path length L0, where L1 represents the totalinterior length of all the header sections of the first header tank, L2represents the total interior length of all the header sections of thesecond header tank, T represents the length of the flat tubes, and Nrepresents the number of the header sections of each header tank, theposition of the refrigerant inlet and the position of the refrigerantoutlet are determined to satisfy the relation 0.8≦LX/L0≦1.2, where Lxrepresents a flow path length for refrigerant which flows into the firstheader tank from the refrigerant inlet, passes through all the headersections and the flat tubes of all the paths, and flows out of therefrigerant outlet.

5) A heat exchanger according to par. 4), wherein each header tankincludes two header sections, and the number of paths is three.

6) A heat exchanger according to par. 4), wherein the amount ofcompressor lubrication oil mixed in a refrigerant to be used is 1 wt. %or less.

According to the heat exchanger of par. 1), when a value represented bya formula {(L1+L2)/2}+(T×2N) is defined as an average flow path lengthL0, where L1 represents the total interior length of all the headersections of the first header tank, L2 represents the total interiorlength of all the header section(s) of the second header tank, Trepresents the length of the flat tubes, and N represents the number ofthe header section(s) of the second header tank, the position of therefrigerant inlet and the position of the refrigerant outlet aredetermined to satisfy the relation 0.8≦LX/L0≦1.2, where Lx represents aflow path length for refrigerant which flows into the first header tankfrom the refrigerant inlet, passes through all the header sections andthe flat tubes of all the paths, and flows out of the refrigerantoutlet. Therefore, the flow path length LX of refrigerant does notchange greatly with the position of a flat tube of each path throughwhich the refrigerant passes, so that the flow rate and flow speed ofrefrigerant become uniform among the flat tubes of each path.Accordingly, the heat radiation amount of the heat exchanger used as agas cooler of a supercritical refrigeration cycle increases as comparedwith the heat exchangers disclosed in Japanese Patent ApplicationLaid-Open Nos. 2003-279194 and No. 2004-138306, respectively.

According to the heat exchanger of par. 3), a decrease in the heatradiation quantity of the heat exchanger used as a gas cooler of asupercritical refrigeration cycle can be prevented. That is, when theamount of compressor lubrication oil mixed in a refrigerant to be usedis in excess of 1 wt. % or less, a large amount of refrigerant flowsthrough a lower portion of each header section and a lower-side flattube of each path, so that the heat radiation quantity may decrease evenwhen the above-described flow path length is generally the same.

According to the heat exchanger of par. 4), when a value represented bya formula {(L1+L2)/2}+{(T×(N+1)} is defined as an average flow pathlength L0, where L1 represents the total interior length of all theheader sections of the first header tank, L2 represents the totalinterior length of all the header sections of the second header tank, Trepresents the length of the flat tubes, and N represents the number ofthe header sections of each header tank, the position of the refrigerantinlet and the position of the refrigerant outlet are determined tosatisfy the relation 0.8≦LX/L0≦1.2, where Lx represents a flow pathlength for refrigerant which flows into the first header tank from therefrigerant inlet, passes through all the header sections and the flattubes of all the paths, and flows out of the refrigerant outlet.Therefore, the flow path length LX of refrigerant does not changegreatly with the position of a flat tube of each path through which therefrigerant passes, so that the flow rate and flow speed of refrigerantbecome uniform among the flat tubes of each path. Accordingly, the heatradiation amount of the heat exchanger used as a gas cooler of asupercritical refrigeration cycle increases as compared with the heatexchangers disclosed in Japanese Patent Application Laid-Open Nos.2003-279194 and No. 2004-138306, respectively.

According to the heat exchanger of par. 6), a decrease in the heatradiation quantity of the heat exchanger used as a gas cooler of asupercritical refrigeration cycle can be prevented. That is, when theamount of compressor lubrication oil mixed in a refrigerant to be usedis in excess of 1 wt. % or less, a large amount of refrigerant flowsthrough a lower portion of each header section and a lower-side flattube of each path, so that the heat radiation quantity may decrease evenwhen the above-described flow path length is generally the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall front view showing a first embodiment of a heatexchanger according to the present invention;

FIG. 2 is a diagram showing flow of refrigerant within the heaterexchanger of FIG. 1;

FIG. 3 is an overall front view showing a second embodiment of the heatexchanger according to the present invention;

FIG. 4 is a diagram showing flow of refrigerant within the heaterexchanger of FIG. 3; and

FIG. 5 is a graph showing results of Test Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will next be described withreference to the drawings.

In the following description, the upper, lower, left-hand, andright-hand sides of FIGS. 1 and 3 will be referred to as “upper,”“lower,” “left,” and “right,” respectively. Further, in the followingdescription, the term “aluminum” encompasses aluminum alloys in additionto pure aluminum.

First Embodiment

FIGS. 1 and 2 show a first embodiment.

FIG. 1 shows the overall structure of a heat exchanger according to thepresent invention. FIG. 2 shows flow of refrigerant within the heaterexchanger.

As shown in FIG. 1, a heat exchanger 1 includes two header tanks 2 and 3formed of aluminum and extending vertically. The header tanks 2 and 3are disposed in parallel and are spaced apart from each other in theleft-right direction. A plurality of flat tubes 4 formed of aluminum arearranged in parallel between the two header tanks 2 and 3 and are spacedapart from one another in the vertical direction. Opposite ends of theflat tubes 4 are connected to the corresponding header tanks 2 and 3.Corrugated fins 6 formed of aluminum are arranged in respectiveair-passing clearances 5 between adjacent flat tubes 4 and at theoutside of the upper-end and lower-end flat tubes 4, and each is brazedto the adjacent flat tube(s) 4. Side plates 7 formed of aluminum arearranged externally of and are brazed to the respective upper-end andlower-end corrugated fins 6.

The right-hand, first header tank 2 includes a plurality of (two in thepresent embodiment) header sections 8 and 9 arranged vertically andseparated by means of a partition 10 located at the center of the tank 2with respect to the vertical direction. The left-hand, second headertank 3 includes a header section(s), which is one fewer in number thanthe header sections 8 and 9 of the first header tank 2; i.e., a singleheader section 11 in the present embodiment, such that the headersection 11 extends from an upper end to a lower end of the second headertank 3 and faces or is opposed to the header sections 8 and 9 of thefirst header tank 2. A refrigerant inlet 12 is provided in theperipheral wall of the upper header section 8 of the first header tank2, and a refrigerant outlet 13 is provided in the peripheral wall of thelower header section 9 of the first header tank 2.

The flat tubes 4 are divided into first and second groups which formfirst and second paths P1 and P2. The interiors of the flat tubes 4belonging to the first group communicate, at their right ends, with theinterior of the upper header section 8 of the first header tank 2, andcommunicate, at their left ends, with an upper portion of the interiorof the header section 11 of the second header tank 3. The interiors ofthe flat tubes 4 belonging to the second group communicate, at theirright ends, with the interior of the lower header section 9 of the firstheader tank 2, and communicate, at their left ends, with a lower portionof the interior of the header section 11 of the second header tank 3.The refrigerant flows in the same direction among the flat tubes 4 whichconstitute the path P1, and flows in the same direction among the flattubes 4 which constitute the path P2. However, the flow direction of therefrigerant flowing through the flat tubes 4 which constitute the pathP1 is opposite that of the refrigerant flowing through the flat tubes 4which constitute the path P2. Although not illustrated in the drawings,each flat tube 4 has a plurality of refrigerant channels formed thereinand arranged in its width direction, and is disposed such that its widthdirection coincides with the direction of flow of air (the directionperpendicular to the sheet of FIG. 1).

When a value obtained by dividing the number of flat tubes 4 whichconstitute each of the paths P1 and P2 by the number of all the tubes 4is defined as a “tube ratio,” the tube ratio of each of the paths P1 andP2 preferably falls within a range of 0.45 to 0.55. Notably, the sum ofthe tube ratio of the path P1 and that of the path P2 becomes 1. Whenthe tube ratio is less than 0.45 or greater than 0.55, an increasedpressure loss may be produced in the flat tubes 4 of one of the paths P1and P2 when the heat exchanger 1 is used as a gas cooler of asupercritical refrigeration cycle in which a CO₂ (carbon dioxide)refrigerant or a like supercritical refrigerant is used and in whichrefrigerant on the high-pressure side is in a supercritical state; i.e.,assumes a pressure in excess of a critical pressure. More preferably,the tube ratio of each of the paths P1 and P2 falls within a range of0.48 to 0.52. In this case as well, the sum of the tube ratio of thepath P1 and that of the path P2 becomes 1.

Here, the refrigerant is assumed to flow within the heat exchanger 1 asshown in FIG. 2. In this case, an average flow path length L0 can bedefined by a formula {(L1+L2)/2}+(T×2N), where L1 represents the sum ofthe interior length Ia of the upper header section 8 and the interiorlength Ib of the lower header section 9 of the first header tank 2(L1=Ia+Ib), L2 represents the interior length Ic of the header section11 of the second header tank 3 (L2=Ic), T represents the length of theflat tubes 4, and N represents the number of the header section 11 ofthe second header tank 3. The position of the refrigerant inlet 12 inthe vertical direction (the longitudinal direction of the header section8) and the position of the refrigerant outlet 13 in the verticaldirection (the longitudinal direction of the header section 9) aredetermined to satisfy the relation 0.8≦LX/L0≦1.2, where Lx represents anactual flow path length for refrigerant which flows into the upperheader section 8 of the first header tank 2 from the refrigerant inlet12, passes through all the header sections 8, 9, and 11 and the flattubes 4 of all the paths P1 and P2, and flows out of the refrigerantoutlet 13.

For example, in a case where the refrigerant inlet 12 is located at aposition indicated by point X1 in FIG. 2, the actual flow path length LXof refrigerant which flows into the upper header section 8 and thenflows through the upper-end flat tube 4 of the first path P1 becomeslonger than the average flow path length L0 by a length Y1. Meanwhile,the actual flow path length LX of refrigerant which flows into the upperheader section 8 and then flows through the lower-end flat tube 4 of thefirst path P1 becomes shorter than the average flow path length L0 bythe length Y1. In view of this, the vertical position of the refrigerantinlet 12 is determined such that the value of (L0+Y1)/L0 and the valueof (L0−Y1)/L0 fall within the range of 0.8 to 1.2. Further, in a casewhere the refrigerant outlet 13 is located at a position indicated bypoint X2 in FIG. 2, the actual flow path length LX of refrigerant whichpasses through the lower-end flat tube 4 of the second path P2 and flowsinto the lower header section 9 becomes longer than the average flowpath length L0 by a length Y2. Meanwhile, the actual flow path length LXof refrigerant which passes through the upper-end flat tube 4 of thesecond path P2 and flows into the lower header section 9 becomes shorterthan the average flow path length L0 by the length Y2. In view of this,the vertical position of the refrigerant outlet 13 is determined suchthat the value of (L0+Y2)/L0 and the value of (L0−Y2)/L0 fall within therange of 0.8 to 1.2.

Second Embodiment

FIGS. 3 and 4 show a second embodiment.

FIG. 3 shows the overall structure of a heat exchanger according to thepresent invention. FIG. 4 shows flow of refrigerant within the heaterexchanger.

In the case of a heat exchanger 20 of the present embodiment, aleft-hand header tank is a first header tank 21, and a right-hand headertank is a second header tank 22. The first header tank 21 includes aplurality of (two in the present embodiment) header sections 24 and 25arranged vertically and separated by means of a partition 23 located ata position higher than the center of the tank 21 with respect to thevertical direction. The second header tank 22 includes header sections,which are equal in number to the header sections 24 and 25 of the firstheader tank 21; i.e., two header sections 27 and 28 arranged verticallyand separated by means of a partition 26 located at a position lowerthan the center of the tank 22 with respect to the vertical direction.The refrigerant inlet 12 is provided in the peripheral wall of the upperheader section 24 of the first header tank 21, and the refrigerantoutlet 13 is provided in the peripheral wall of the lower header section28 of the second header tank 22.

The flat tubes 4 are divided into first through third groups which formfirst through third paths P1 to P3, respectively. The interiors of theflat tubes 4 belonging to the first group communicate, at their leftends, with the interior of the upper header section 24 of the firstheader tank 21, and communicate, at their right ends, with an upperportion of the interior of the upper header section 27 of the secondheader tank 22. The interiors of the flat tubes 4 belonging to thesecond group communicate, at their left ends, with an upper portion ofthe interior of the lower header section 25 of the first header tank 21,and communicate, at their right ends, with a lower portion of theinterior of the upper header section 27 of the second header tank 22.The interiors of the flat tubes 4 belonging to the third groupcommunicate, at their left ends, with a lower portion of the interior ofthe lower header section 25 of the first header tank 21, andcommunicate, at their right ends, with the interior of the lower headersection 28 of the second header tank 22. The refrigerant flows in thesame direction among the flat tubes 4 which constitute the path P1, inthe same direction among the flat tubes 4 which constitute the path P2,and in the same direction among the flat tubes 4 which constitute thepath P3. However, the flow direction of the refrigerant flowing throughthe flat tubes 4 which constitute the path P1 is opposite that of therefrigerant flowing through the flat tubes 4 which constitute the pathP2; and the flow direction of the refrigerant flowing through the flattubes 4 which constitute the path P2 is opposite that of the refrigerantflowing through the flat tubes 4 which constitute the path P3.

When a value obtained by dividing the number of flat tubes 4 whichconstitute each of the paths P1, P2, and P3 by the number of all thetubes 4 is defined as a “tube ratio,” the tube ratio of each of thepaths P1, P2, and P3 preferably falls within a range of 0.3 to 0.4. Whenthe tube ratio is less than 0.3 or greater than 0.4, an increasedpressure loss may be produced in the flat tubes 4 of one of the pathsP1, P2, and P3 when the heat exchanger 20 is used as a gas cooler of asupercritical refrigeration cycle in which a CO₂ (carbon dioxide)refrigerant or a like supercritical refrigerant is used and in whichrefrigerant on the high-pressure side is in a supercritical state; i.e.,assumes a pressure in excess of a critical pressure. More preferably,the tube ratio of each path falls within a range of 0.32 to 0.34.

Other structural features are identical with those of the firstembodiment. Like members are denoted by like reference numerals, andtheir description will not be repeated.

Here, the refrigerant is assumed to flow within the heat exchanger 20 asshown in FIG. 4. In this case, an average flow path length L0 can bedefined by a formula {(L1+L2)/2}+{T×(N+1)}, where L1 represents the sumof the interior length Id of the upper header section 24 and theinterior length Ie of the lower header section 25 of the first headertank 21 (L1=Id+Ie), L2 represents the sum of the interior length If ofthe upper header section 27 and the interior length Ig of the lowerheader section 28 of the second header tank 22 (L2=If+Ig), T representsthe length of the flat tubes 4, and N represents the number of theheader sections 24 and 25 (27 and 28) of each header tank 21 (22). Theposition of the refrigerant inlet 12 in the vertical direction (thelongitudinal direction of the header section 24) and the position of therefrigerant outlet 13 in the vertical direction (the longitudinaldirection of the header section 28) are determined to satisfy therelation 0.8≦LX/L0≦1.2, where Lx represents an actual flow path lengthfor refrigerant which flows into the upper header section 24 of thefirst header tank 21 from the refrigerant inlet 12, passes through allthe header sections 24, 25, 27, and 28 and the flat tubes 4 of the pathsP1, P2, and P3, and flows out of the refrigerant outlet 13.

For example, in a case where the refrigerant inlet 12 is located at aposition indicated by point X3 in FIG. 4, the actual flow path length LXof refrigerant which flows into the upper header section 24 of the firstheader tank 21 and then flows through the upper-end flat tube 4 of thefirst path P1 becomes longer than the average flow path length L0 by alength Y3. Meanwhile, the actual flow path length LX of refrigerantwhich flows into the upper header section 24 of the first header tank 21and then flows through the lower-end flat tube 4 of the first path P1becomes shorter than the average flow path length L0 by the length Y3.In view of this, the vertical position of the refrigerant inlet 12 isdetermined such that the value of (L0+Y3)/L0 and the value of (L0−Y3)/L0fall within the range of 0.8 to 1.2. Further, in a case where therefrigerant outlet 13 is located at a position indicated by point X4 inFIG. 4, the actual flow path length LX of refrigerant which passesthrough the lower-end flat tube 4 of the third path P3 and flows intothe lower header section 28 of the second header tank 22 becomes longerthan the average flow path length L0 by a length Y4. Meanwhile, theactual flow path length LX of refrigerant which passes through theupper-end flat tube 4 of the third path P3 and flows into the lowerheader section 28 of the second header tank 22 becomes shorter than theaverage flow path length L0 by the length Y4. In view of this, thevertical position of the refrigerant outlet 13 is determined such thatthe value of (L0+Y4)/L0 and the value of (L0−Y4)/L0 fall within therange of 0.8 to 1.2.

Each of the heat exchangers 1 and 20 of the first and second embodimentsare preferably used as a gas cooler of a supercritical refrigerationcycle which includes a compressor, the gas cooler, an evaporator, anaccumulator serving as a gas-liquid separator, an expansion valveserving as a pressure reducing device, and an intermediate heatexchanger for exchanging heat between high temperature, high pressurerefrigerant flowing out of the gas cooler and low temperature, lowpressure refrigerant flowing out of the evaporator and then passingthrough the accumulator, and which uses a CO₂ supercritical refrigerant.In such a supercritical refrigeration cycle, preferably, the amount ofcompressor lubrication oil mixed in the supercritical refrigerant is 1wt. % or less.

The refrigeration cycle is installed in a vehicle; for example, in anautomobile, as a car air conditioner. Although CO₂ is used as asupercritical refrigerant of a supercritical refrigeration cycle, therefrigerant is not limited thereto, and ethylene, ethane, nitrogenoxide, or the like may alternatively be used.

Next, a test example performed by use of the heat exchanger of the firstembodiment will be described.

TEST EXAMPLE 1

A heat exchanger used in the test was configured such that a heatexchanger core section composed of the flat tubes 4 and the corrugatefins 6 has a height Hc of 380 mm and a width Wc of 660 mm; the width ofthe flat tubes 4 is 16 mm; the total number of the flat tubes 4 is 51;the number of the flat tubes 4 of the first path P1 is 26 (tube ratio:0.51); and the number of the flat tubes 4 of the second path P2 is 25(tube ratio: 0.49). The heat radiation quantity of the heat exchangerwas obtained, while the ratio LX/L0; the ratio of the above-describedflow path length Lx to the above-described average flow path length L0,was varied, under the conditions that inlet air temperature (temperatureof air flowing into the heat exchange core section) was 35 to 40° C.;and front-face air speed (flow speed of air flowing into the heatexchange core section) was 1.5 to 2.5 m/s.

FIG. 5 shows the relation between the ratio LX/L0 and the heat radiationquantity. The results shown in FIG. 5 demonstrate that the heatexchanger has an excellent heat radiation performance when the ratioLX/L0 falls within the range of 0.8 to 1.2.

1. A heat exchanger comprising first and second header tanks disposed apart from each other; and a plurality of flat tubes disposed between the first and second header tanks such that the flat tubes are spaced apart from one another in the longitudinal direction of the header tanks, opposite end portions of the flat tubes being connected to the respective header tanks, the first header tank including a plurality of header sections arranged in the longitudinal direction of the first header tank, the second header tank including a single header section or a plurality of header sections which are one fewer in number than the header sections of the first header tank, the single header section or each of the header sections facing two adjacent header sections of the first header tank, a refrigerant inlet being provided at a header section of the first header tank located at one end with respect to the longitudinal direction, a refrigerant outlet being provided at another header section of the first header tank located at the other end with respect to the longitudinal direction, the flat tubes being divided into groups each of which includes a plurality of flat tubes arranged in the longitudinal direction of the header tanks and which form paths which are equal in number to the header sections of the first header tank, wherein when a value represented by a formula {(L1+L2)/2}+(T×2N) is defined as an average flow path length L0, where L1 represents the total interior length of all the header sections of the first header tank, L2 represents the total interior length of all the header section(s) of the second header tank, T represents the length of the flat tubes, and N represents the number of the header section(s) of the second header tank, the position of the refrigerant inlet and the position of the refrigerant outlet are determined to satisfy the relation 0.8≦LX/L0≦1.2, where Lx represents a flow path length for refrigerant which flows into the first header tank from the refrigerant inlet, passes through all the header sections and the flat tubes of all the paths, and flows out of the refrigerant outlet.
 2. A heat exchanger according to claim 1, wherein the first header tank includes two header sections, the second header tank includes a single header section, and the number of paths is two.
 3. A heat exchanger according to claim 1, wherein the amount of compressor lubrication oil mixed in a refrigerant to be used is 1 wt. % or less.
 4. A heat exchanger comprising first and second header tanks disposed apart from each other; and a plurality of flat tubes disposed between the first and second header tanks such that the flat tubes are spaced apart from one another in the longitudinal direction of the header tanks, opposite end portions of the flat tubes being connected to the respective header tanks, the first header tank including a plurality of header sections arranged in the longitudinal direction of the first header tank, the second header tank including header sections which are equal in number to the header sections of the first header tank and which are arranged in the longitudinal direction of the second header tank, a refrigerant inlet being provided at a header section of the first header tank located at one end with respect to the longitudinal direction, a refrigerant outlet being provided at a header section of the second header tank located at the opposite end with respect to the longitudinal direction, the flat tubes being divided into groups each of which includes a plurality of flat tubes arranged in the longitudinal direction of the header tanks and which form paths which are one greater in number than the header sections of each header tank, wherein when a value represented by a formula {(L1+L2)/2}+{(T×(N+1)) is defined as an average flow path length L0, where L1 represents the total interior length of all the header sections of the first header tank, L2 represents the total interior length of all the header sections of the second header tank, T represents the length of the flat tubes, and N represents the number of the header sections of each header tank, the position of the refrigerant inlet and the position of the refrigerant outlet are determined to satisfy the relation 0.8≦LX/L0≦1.2, where Lx represents a flow path length for refrigerant which flows into the first header tank from the refrigerant inlet, passes through all the header sections and the flat tubes of all the paths, and flows out of the refrigerant outlet.
 5. A heat exchanger according to claim 4, wherein each header tank includes two header sections, and the number of paths is three.
 6. A heat exchanger according to claim 4, wherein the amount of compressor lubrication oil mixed in a refrigerant to be used is 1 wt. % or less. 